In recent years, along with widespread use of OA equipment, communications equipment, and the like, problems associated with electromagnetic waves generated from such equipment have been receiving attention. There have been concerns that electromagnetic waves may have adverse effects on the human body, and also there have been occurrences of malfunction and the like of precision equipment caused by electromagnetic waves.
Electromagnetic-wave-shielding light-transmitting window members have been developed as front filters for PDPs in OA equipment and put into practical use. Such window members are also used in sites installed with precision equipment, such as hospitals and laboratories, to protect the precision equipment from electromagnetic waves generated from cellular phones and the like.
A known electromagnetic-wave-shielding light-transmitting window member mainly includes a conductive mesh member, such as a wire mesh, disposed between and integrated with transparent substrates, such as acrylic plates.
In general, conductive meshes used for known electromagnetic-wave-shielding light-transmitting window members have a wire diameter of 10 to 500 μm, a mesh size of about 5 to 500 mesh, and an open area ratio of less than 75%.
In the known conductive meshes which have been used, in general, conductive meshes composed of thicker conductive fibers are coarser while conductive meshes composed of thinner conductive fibers are finer. The reason for this is that although a coarse mesh can be produced with thick fibers, it is extremely difficult to produce a coarse mesh using thin fibers.
Accordingly, known electromagnetic-wave-shielding light-transmitting window members including such conductive meshes have a light transmittance of at most about 70%, and thus it is not possible to achieve satisfactory light transmittance, which is disadvantageous.
Furthermore, the known conductive meshes readily cause moire fringes (interference fringes) in connection with pixel pitches of light-emitting panels on which electromagnetic-wave-shielding light-transmitting window members are mounted, which is also disadvantageous.
In order to overcome such problems, formation of a conductive pattern using a pattern printing method has been proposed. For example, Japanese Unexamined Patent Application Publication No. 11-170420 proposes a method including forming a printed pattern on a surface of a transparent substrate using a paste containing an electroless plating catalyst, and depositing a conductive material on the printed pattern by electroless plating to form a conductive pattern. In pattern printing, it is possible to obtain a desired conductive pattern by forming a printed pattern containing an electroless plating catalyst into a desired shape and depositing a plating layer thereon. Thus, the pattern printing method allows a markedly high degree of freedom in designing the line width, spacing, and network shape compared with use of conductive meshes, and it is possible to easily form a lattice-like conductive pattern with thin lines and a high open area ratio, for example, at a line width of 200 μm or less and an open area ratio of 75% or more. In an electromagnetic-wave-shielding light-transmitting window member including such a coarse conductive pattern with thin lines, satisfactory light transmittance can be achieved and the moire phenomenon can be prevented. Here, the open area ratio is calculated from the mesh line width and the number of lines present per inch in width.
In the method described in Japanese Unexamined Patent Application Publication No. 11-170420, specifically, a printing paste which contains particles of an electroless plating catalyst, a resin component as a binder, a solvent, and other additives is pattern-printed, and after the resulting printed pattern is cured, a plating layer is deposited by electroless plating.
Japanese Unexamined Patent Application Publication No. 11-170421 describes a method including forming a pattern containing an electroless plating catalyst not by a printing method but by exposure through a photomask, and depositing a plating layer on the resulting pattern by electroless plating to form a conductive pattern.
In the method described in Japanese Unexamined Patent Application Publication No. 11-170420, since the plating layer is deposited by electroless plating on the printed pattern, it is necessary to blend particles of an electroless plating catalyst, at a high content, to a printing paste (i.e., high-content blending). Use of the printing paste including high-content blended catalyst particles gives rise to the following problems:
(1) Since a large amount of catalyst particles is contained, it is difficult to adjust the viscosity of the printing paste, resulting in difficulty in forming a micropattern with high accuracy.
(2) Since a large amount of catalyst particles is contained, the amount of the resin component which is a binder is relatively small, and in some cases, adhesion between the transparent substrate and the printed pattern, and further adhesion with the conductive pattern, may be degraded.
(3) Since the catalyst particles are expensive, the increase in the blending amount thereof leads to soaring of the material cost.
In the method described in Japanese Unexamined Patent Application Publication No. 11-170420, even if the particles of the electroless plating catalyst are blended at a high content to the printing paste, unless the particles of the catalyst are exposed to the surface of the printed pattern formed by pattern printing, it is not possible to deposit a plating layer by electroless plating on the surface of the printed pattern.
In the method according to Japanese Unexamined Patent Application Publication No. 11-170420, by increasing the ratio of the catalyst particles to the binder to a certain degree, the catalyst particles are exposed after pattern printing. In such a printing paste, in order to achieve a viscosity suitable for printing, a considerable amount of a solvent must be added. In the printing paste containing the solvent, a heat drying process is necessary after printing, and dripping during heating and a length of tact time (heating time) can cause problems for operation.
Examples of the resin include a thermosetting resin which is cured by heating and an ultraviolet-curable resin which is cured by irradiation of ultraviolet light. The ultraviolet-curable resin is advantageous in that the curing rate is high and productivity is high. In the method according to Japanese Unexamined Patent Application Publication No. 11-170421 in which a photoresist process is employed, an ultraviolet-curable resin is used. When an ultraviolet-curable resin containing particles of an electroless plating catalyst is cured by irradiation of ultraviolet light, a part of the ultraviolet light applied is absorbed by the catalyst particles. Therefore, the irradiation efficiency of ultraviolet light is low, and the curing efficiency of the resin is low. In particular, inner layers are not easily cured. This problem becomes more conspicuous when the catalyst particles are blended at a high content. In order to prevent the catalyst particles from absorbing ultraviolet light, it is necessary to select catalyst particles with high transmittance with respect to ultraviolet light. In such a case, catalyst species that can be used are restricted.